The present invention generally relates to a sintered silicon nitride product. The present invention also relates to methods for making a sintered silicon nitride product.
Sintered silicon nitride compositions represent a group of ceramics useful as engineering components for high temperature applications, such as in gas turbine engines. Some prior art sintered silicon nitride products have adequate mechanical properties and oxidation resistance at temperatures in the range of 900° C. to 1400° C. However, such prior art products may exhibit major deficiencies in advanced gas turbine applications in which the turbine inlet temperature may exceed 1500° C.
U.S. Pat. No. 4,401,768 to Morgan discloses a sintered silicon nitride prepared from a mixture of silicon nitride, silicon dioxide (silica), and scandium oxide (scandia). The sintered silicon nitride was produced by hot pressing at 1800° C. to provide a product consisting of silicon nitride, silicon oxynitride, and scandium disilicate. Morgan does not disclose properties such as fracture toughness or oxidation resistance at a temperature of 1500° C.
U.S. Pat. No. 5,804,523 to Oda et al., discloses the production of sintered silicon nitride from a mixture of silicon nitride, silica, and an oxide of a Group 3a (Re) element, the Group 3a element at least including lutetium (Lu). The composition of the sintered product may contain intergranular crystalline phases of a Re2Si2O7 (disilicate) or a Re2SiO5 (monosilicate). Oda et al. discloses fracture toughness only in the range of about 4.5 to 6.5 MPa.m0.5.
As can be seen, there is a need for a strong and reliable silicon nitride ceramic with improved mechanical stability and a high level of toughness at high temperatures. There is a further need for a sintered silicon nitride product which exhibits resistance to corrosion and oxidation at high temperatures. The present invention provides such materials, and processes of making the same, as will be described in enabling detail hereinbelow.